The invention relates to a high-pressure discharge lamp comprising a ceramic discharge vessel which encloses a discharge space which is provided with an ionizable filling comprising metal halide and in which a first and a second electrode are arranged, which discharge vessel comprises, on either side of a central zone extending between the electrodes, a first and a second end zone which are connected to the central zone, which each surround with little clearance a current supply conductor connected to a respective electrode, and in which a seal of ceramic sealing compound is provided through which said current supply conductor issues to the exterior, in which lamp at least the first end zone has an external diameter smaller than the smallest external diameter of the central zone and the current supply conductor through the first end zone has a halide-resistant portion facing the discharge space and a portion which is permeable to hydrogen and oxygen remote from the discharge space.
Such a lamp is known from U.S. Pat. No. 4,409,517. The term "ceramic discharge vessel" in the present description and claims is understood to mean a discharge vessel of a refractory material such as monocrystalline metal oxide, for example sapphire, polycrystalline metal oxide, for example translucent gastight aluminium oxide (DGA), yttrium-aluminium garnet (YAG) or yttrium oxide (YOX), or polycrystalline non-oxidic material such as aluminium nitride (AlN). The term "halide resistant" means that no or substantially no corrosive attack by halides and free halogens takes place under the conditions prevailing in the discharge space during lamp operation. The term "little clearance" means that the space remaining between the end zone and the current supply conductor issuing through it is at least 5 .mu.m and at most one fourth of the internal diameter of the end zone, but not more than approximately 200 .mu.m. So the diameter of the current supply conductor therein is at least equal to half the internal diameter of the end zone. In the known lamp, a metal bush forming a current supply conductor is passed through each of the end zones of the discharge vessel. The space remaining between the bush and the end zone is entirely filled with a ceramic sealing compound. Niobium or tantalum is used as the material for the current supply conductor because these metals have coefficients of expansion, averaged over the temperature range which the end zone experiences after the lamp has been switched on from an idle state, which correspond substantially to those of the ceramic materials from which the discharge vessel is manufactured. A disadvantage of the said metals, however, is that they are not halide resistant. Accordingly, the current supply conductor issuing into the discharge vessel through the first end zone in the known lamp is provided with a cover of halide-resistant material such as molybdenum or tungsten at a portion situated inside the discharge space.
It has been found to be difficult as a rule to avoid that hydrogen enters the discharge vessel during the manufacture of high-pressure discharge lamps comprising metal halide, or that hydrogen is evolved in a later stage through dissociation of water present in the discharge vessel, for example, absorbed in the metal halide salts. Small quantities of hydrogen can already cause a strong rise in the ignition voltage and re-ignition voltage of the lamp. It is also possible for parasitic reactions with oxygen to occur, which can lead to a black discolouration of the discharge vessel and also to a rise in the (re-)ignition voltage. A ratio of re-ignition voltage to lamp voltage greater than 2 involves the risk of the lamp extinguishing during operation on a conventional lamp supply. To counteract these disadvantages, the current supply conductor through the second end zone is entirely made of niobium or tantalum in the known lamp. This is because these metals are highly permeable to hydrogen and oxygen. These gases can leave the discharge vessel through this current supply conductor.
To prevent attacks on the current supply conductor issuing from the second end zone in a lamp of this construction, it is necessary to operate the lamp in vertical or substantially vertical position so that a separation takes place in the discharge vessel whereby the halides and free halogens are present mainly in the end zone situated in the upper part. A disadvantage is also that the use of the construction of the known lamp is usually only possible for lamps having a sufficiently long and narrow discharge vessel. Lamps having a comparatively short and wide discharge vessel are usually so operated that the fill ingredients have a comparatively high vapour pressure to render possible the realisation of a sufficiently high lamp voltage in spite of the small discharge vessel length. Under these circumstances there is a risk that, given a vertical position of the discharge vessel, a too strong convection flow will take place for achieving a separation and thus for preventing attacks on the current supply conductor which issues through the second end zone.